Ciliopathies are a spectrum of diseases resulting from defects in primary cilia function affecting 1:800 people. Primary cilia are microtubule based organelles found on almost all cells and crucial for proper signal transduction of a number of molecular pathways. Ciliopathies affect a wide range of tissues including the nervous system, craniofacial tissues, skeleton, kidneys, lungs and digestive organs. These manifest as both congenital and adult-onset defects. Genetic studies of ciliopathy patients show TTC21B (tetratricopeptide repeat domain-containing protein1B) is the most commonly mutated cilia gene identified to date. In addition to defects associated with loss of just TTC21B, mutations in trans with a number of other ciliary genes lead to ciliopathies. We have recently shown loss of Ttc21b in the mouse leads to perinatal lethality and organogenesis defects. We also note some of these phenotypes are dependent on the specific inbred mouse strain background. The TTC21B protein is large with many protein-protein interaction domains and important for intraflagellar transport and regulating signal transduction in the cilium. All of these data together lead us to the central hypothesis that TTC21B serves as a network hub for scaffolding and trafficking activities essential for proper cilia form and function. The goal of this application is identify genes and proteins interacting with Ttc21b: the Ttc21b interactome, and begin to understand how these interact in the cell. The rationale for the project is that a more complete understanding of how TTC21B acts is likely to give insight to a range of ciliopathies. We will address this hypothesis and achieve these goals with the following three specific aims: 1) identify chromosomal regions containing genes modifying the Ttc21bnull/null phenotype in the B6 and FVB mouse strains, 2) identify novel genetic interactions with Ttc21b using a forward genetic approach, and 3) study functional mechanisms of genes interacting with Ttc21b. The first aim will utilize a QTL analysis to identify loci regulating the strain specific phenotypes we see in Ttc21bnull/null embryos. The second aim will take a forward genetic, ENU mutagenesis approach to identify novel interactions with TTC21B in an unbiased manner. The third aim will recapitulate interactions identified in humans or previously identified in mouse. After verifying these interactions yield ciliopathy phenotype(s), we will perform further analyses in vitro and in vivo to study the molecular mechanisms of ciliary dysfunction. These studies will focus on ciliary trafficking and Shh signal transduction. The significance of this project is that these studies wil together dramatically increase our understanding of how TTC21B acts within the primary cilium and why perturbation of function leads to ciliopathic disease. These studies will fill an important gap in our knowledge and identify possible areas for therapeutic intervention. These advances are not specific to TTC21B but are likely going to be largely applicable to multiple areas of primary cilia biology inter- est. The innovation of this project lies in the application of unbiase genetic techniques to identify the Ttc21b interactome in close concert with solid molecular studies to determine the underlying mechanism(s).